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Lesson 4: Displaying Public Health Data

Summary, References, and Websites

Much work has been done on other graphical methods of presentation.(33) One of the more creative is face plots.(34) Originally developed by Chernoff,(35) these give a way to display n variables on a two-dimensional surface. For instance, suppose you have several variables (x, y, z, etc.) that you have collected on each of n people, and for purposes of this illustration, suppose each variable can have one of 10 possible values. We can let x be eyebrow slant, y be eye size, z be nose length, etc. The figures below show faces produced using 10 characteristics — head eccentricity, eye size, eye spacing, eye eccentricity, pupil size, eyebrow slant, nose size, mouth shape, mouth size, and mouth opening) — each assigned one of 10 possible values.

To convey the messages of epidemiologic findings, you must first select the best illustration method. Tables are commonly used to display numbers, rates, proportions, and cumulative percents. Because tables are intended to communicate information, most tables should have no more than two variables and no more than eight categories (class intervals) for any variable. Printed tables should be properly titled, labeled, and referenced; that is, they should be able to stand alone if separated from the text.

Tables can be used with either nominal or continuous ordinal data. Nominal variables such as sex and state of residence have obvious categories. For continuous variables that do not have obvious categories, class intervals must be created. For some diseases, standard class intervals for age have been adopted. Otherwise a variety of methods are available for establishing reasonable class intervals. These include class intervals with an equal number of people or observations in each; class intervals with a constant width; and class intervals based on the mean and standard deviation.

Graphs can visually communicate data rapidly. Arithmetic-scale line graphs have traditionally been used to show trends in disease rates over time. Semilogarithmic-scale line graphs are preferred when the disease rates vary over two or more orders of magnitude. Histograms and frequency polygons are used to display frequency distributions. A special type of histogram known as an epidemic curve shows the number of cases by time of onset of illness or time of diagnosis during an epidemic period. The cases may be represented by squares that are stacked to form the columns of the histogram; the squares may be shaded to distinguish important characteristics of cases, such as fatal outcome.

Simple bar charts and pie charts are used to display the frequency distribution of a single variable. Grouped and stacked bar charts can display two or even three variables.

Spot maps pinpoint the location of each case or event. An area map uses shading or coloring to show different levels of disease numbers or rates in different areas.

The final pages of this lesson provide guidance in the selection of illustration methods and construction of tables and graphs. When using each of these methods, it is important to remember their purpose: to summarize and to communicate. Even the best method must be constructed properly or the message will be lost. Glitzy and colorful are not necessarily better; sometimes less is more!

Guide to Selecting a Graph or Chart to Illustrate Epidemiologic Data

Type of Graph or Chart

When to Use

Arithmetic scale line graph

Show trends in numbers or rates over time

Semilogarithmic scale line graph

Display rate of change over time; appropriate for values ranging over more than 2 orders of magnitude

Histogram

Show frequency distribution of continuous variable; for example, number of cases during epidemic (epidemic curve) or over longer period of time

Frequency polygon

Show frequency distribution of continuous variable, especially to show components

Cumulative frequency

Display cumulative frequency for continuous variables

Scatter diagram

Plot association between two variables

Simple bar chart

Compare size or frequency of different categories of a single variable

Grouped bar chart

Compare size or frequency of different categories of 2 4 series of data

Stacked bar chart

Compare totals and illustrate component parts of the total among different groups

Checklist for Constructing Printed Tables

Title

Does the table have a title?

Does the title describe the objective of the data display and its content, including subject, person, place, and time?

Is the title preceded by the designation “Table #''? (“Table'' is used for typed text; “Figure'' is used for graphs, maps, and illustrations. Separate numerical sequences are used for tables and figures in the same document (e.g., Table 4.1, Table 4.2; Figure 4.1, Figure 4.2).

Rows and Columns

Is each row and column labeled clearly and concisely?

Are the specific units of measurement shown? (e.g., years, mg/dl, rate per 100,000).

Are the categories appropriate for the data?

Are the row and column totals provided?

Footnotes

Are all codes, abbreviations, or symbols explained?

Are all exclusions noted?

If the data are not original, is the source provided?

If source is from website, is complete address specified; and is current, active, and reference date cited?

Checklist for Constructing Printed Graphs

Title

Does the graph or chart have a title?

Does the title describe the content, including subject, person, place, and time?

Is the title preceded by the designation “Figure #''? (“Table'' is used for typed text; “Figure'' is used for graphs, charts, maps, and illustrations. Separate numerical sequences are used for tables and figures in the same document (e.g., Table 1, Table 2; Figure 1, Figure 2).

Axes

Is each axis labeled clearly and concisely?

Are the specific units of measurement included as part of the label? (e.g., years, mg/dl, rate per 100,000)

Are the scale divisions on the axes clearly indicated?

Are the scales for each axis appropriate for the data?

Does the y axis start at zero?

If a scale break is used with an arithmetic-scale line graph, is it clearly identified?

Has a scale break been used with a histogram, frequency polygon, or bar chart? (Answer should be NO!)

Are the axes drawn heavier than the other coordinate lines?

If two or more graphs are to be compared directly, are the scales identical?

Grid Lines

Does the figure include only as many grid lines as are necessary to guide the eye? (Often, these are unnecessary.)

Data plots

Does the table have a title?

Are the plots drawn clearly?

Are the data lines drawn more heavily than the grid lines?

If more than one series of data or components is shown, are they clearly distinguishable on the graph?

Is each series or component labeled on the graph, or in a legend or key?

If color or shading is used on an area map, does an increase in color or shading correspond to an increase in the variable being shown?

Is the main point of the graph obvious, and is it the point you wish to make?

Footnotes

Are all codes, abbreviations, or symbols explained?

Are all exclusions noted?

If the data are not original, is the source provided?

Visual Display

Does the figure include any information that is not necessary?

Is the figure positioned on the page for optimal readability?

Do font sizes and colors improve readability?

Guide to Preparing Projected Slides

Legibility (make sure your audience can easily read your visuals)

When projected, can your visuals be read from the farthest parts of the room?

Simplicity (keep the message simple)

Have you used plain words?

Is the information presented in the language of the audience?

Have you used only key words?

Have you omitted conjunctions, prepositions, etc.?

Is each slide limited to only one major idea/concept/theme?

Is the text on each slide limited to 2 or 3 colors (e.g., 1 color for title, another for text)?

Are there no more than 6–8 lines of text and 6–8 words per line?

Color

Colors have an impact on the effect of your visuals. Use warm/hot colors to emphasize, to highlight, to focus, or to reinforce key concepts. Use cool/cold colors for background or to separate items. The following table describes the effect of different colors.

Hot

Warm

Cool

Cold

Colors:

RedBright orangeBright yellowBright gold

Light orangeLight yellowLight goldBrowns

Light blueLight greenLight purpleLight gray

Dark blueDark greenDark purpleDark gray

Effect:

Exciting

Mild

Subdued

Somber

Are you using the best color combinations? The most important item should be in the text color that has the greatest contrast with its background. The most legible color combinations are:

Black on yellow

Black on white

Dark Green on white

Dark Blue on white

White on dark blue (yellow titles and white text on a dark blue background is a favorite choice among epidemiologists)

Restrict use of red except as an accent.

Accuracy

Slides are distracting when mistakes are spotted. Have someone who has not seen the slide before check for typos, inaccuracies, and errors in general.

References

Koschat MA. A case for simple tables. The American Statistician 2005;59:31–40.

Centers for Disease Control and Prevention. Sexually Transmitted Disease Surveillance, 2002. Atlanta, GA: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, September 2003.

Pierchala C. The choice of age groupings may affect the quality of tabular presentations. ASA Proceedings of the Joint Statistical Meetings; 2002; Alexandria, VA: American Statistical Association; 2002:2697–702.